Unity 贴图叠加，添加水印

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Shader "Custom/AddWatermarkShader"
{
    Properties
    {
        _MainTex("Main Texture", 2D) = "white" {}
        _WatermarkTex("Watermark Texture", 2D) = "white" {}
        _WatermarkPosition("Watermark Position", Vector) = (0, 0, 122, 34)
    }

    SubShader
    {
        Tags{ "Queue" = "Transparent" "IgnoreProjector" = "True" "RenderType" = "Transparent" }

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };

            sampler2D _MainTex;
            float4 _MainTex_TexelSize;

            sampler2D _WatermarkTex;
            float4 _WatermarkTex_TexelSize;

            float4 _WatermarkPosition;

            v2f vert(appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                return o;
            }

            fixed4 frag(v2f i) : SV_Target
            {
                //计算水印贴图相对于底图的尺寸
                float2 ratio = float2(_WatermarkTex_TexelSize.z / _MainTex_TexelSize.z, _WatermarkTex_TexelSize.w / _MainTex_TexelSize.w);

                if (i.uv.x >= _WatermarkPosition.x && i.uv.x <= _WatermarkPosition.x + ratio.x &&
                    i.uv.y >= _WatermarkPosition.y && i.uv.y <= _WatermarkPosition.y + ratio.y)
                {
                    float2 watermarkUV = (i.uv - _WatermarkPosition.xy) / float2(ratio.x, ratio.y);
                    fixed4 watermarkColor = tex2D(_WatermarkTex, watermarkUV);
                    return watermarkColor;
                }
                else
                {
                    fixed4 mainColor = tex2D(_MainTex, i.uv);
                    return mainColor;
                }
            }
            ENDCG
        }
    }
}